Head morphology and electrosensory pore distribution of carcharhinid and sphyrnid sharks

Selection to maximize electroreceptive search area might have driven evolut ion of the cephalofoil head morphology of hammerhead sharks (family Sphyrni dae). The enhanced electrosensory hypothesis predicts that the wider head o f sphyrnid sharks necessitates a greater number of electrosensory pores to maintain a comparable pore density. Although gross head morphology clearly differs between sphyrnid sharks and their closest relatives the carcharhini ds, a quantitative examination is lacking. Head morphology and the distribu tion of electrosensory pores were compared between a carcharhinid, Carcharh inus plumbeus, and two sphyrnid sharks, Sphyrna lewini and S. tiburo. Both sphyrnids had greater head widths than the carcharhinid, although head surf ace area and volume did not differ between the three species. The raked hea d morphology of neonatal S. lewini pups, presumably an adaptation to facili tate parturition, becomes orthogonal to the body axis immediately post-part urition whereas this change is much less dramatic for the other two species . The general pattern of electrosensory pore distribution on the head is co nserved across species despite the differences in gross head morphology. Sp hyrna lewini has a mean of 3067 +/- 158.9 SD pores, S. tiburo has a mean of 2028 +/- 96.6 SD pores and C. plumbeus has a mean of 2317 +/- 126.3 SD por es and the number of pores remains constant with age. Sphyrnids have a grea ter number of pores on the ventral surface of the head whereas C. plumbeus has an even distribution on dorsal and ventral surfaces. The greater number of pores distributed on a similar surface area provides S. lewini pups wit h a higher density of electrosensory pores per unit area compared to C. plu mbeus pups. The greater number of ampullae, the higher pore density and the larger sampling area of the head combine to provide hammerhead sharks with a morphologically enhanced electroreceptive capability compared to compara bly sized carcharhinids.